Aqueous two-phase systems (ATPSs), or all-aqueous emulsions, are formed by dissolving two incompatible solutes in water above the critical concentrations for phase separation. These incompatible solutes can redistribute in water and form immiscible aqueous phases, if the reduction in enthalpy is sufficient to overcome the energy cost associated with the increased entropy. This often requires each solute species of an ATPS to interact more strongly with itself than with the other species, leading to the segregation of solute of the same species and phase separation. However, the segregation is incomplete and each phase usually still contains a small amount of molecules of the other species.
For example, in the equilibrium phase diagram of the PEG/dextran/H2O system, the dextran-rich phase contains a minute amount of PEG; similarly, the PEG-rich phase also contains dextran. Compositions of equilibrium phases vary with the molecular weight of incompatible solutes, temperature and other salt additives. The incomplete separation leads to lower mixing entropy than a complete separation, and thus reduces the energetic cost of phase separation.
Immiscible phases formed by the phase separation of ATPSs are free of organic solvents; thus they are biocompatible and eco-friendly in biomedical research studies and applications. In the synthesis of biomaterials, such as protein microspheres and hydrogel beads, organic solvents are usually involved. Upon solidifying the dispersed phase to form solid materials, organic solvents must be extracted by repeated washing. These tedious steps to remove the organic phases can be avoided when ATPSs are used to form emulsions. Moreover, when protein solutions are exposed to the oil phases, denaturation of proteins often occurs at the water-oil (w/o) interface, reducing the bioactivity of the proteins. The use of ATPS can avoid the detrimental effects of organic solvents to the bioactivity of proteins and the viability of cells.
Traditional methods for fabricating all-aqueous emulsions, such as vortex mixing and homogenization, do not allow for control of the sizes and/or structures of the resultant all-aqueous emulsions. To overcome these limitations, microfluidics has been investigated for the preparation of all-aqueous emulsions. While microfluidic devices enable the generation of water-in-oil (w/o) or oil-in-water (o/w) emulsions with high monodispersity and control over droplet shapes and structures, formation of monodisperse w/w emulsion is not easily achieved in typical microfluidic channels owing to the low interfacial tension of such systems. Breakup of the jet in the low interfacial tension systems usually results in polydisperse droplets because of the large Capillary and Weber numbers.
Attempts to overcome the limitations of microfluidic approaches have been explored. For example, perturbation has been investigated as a means for producing all aqueous emulsions. To induce the breakup of a w/w jet, a vibration source is introduced in the system to generate initial corrugations on the surface of a w/w jet. These vibrations can be incorporated into the microfluidic devices by embedding a piezoelectric actuator in the channel wall or by using a mechanical vibrator that squeezes the soft tubing connected to the incoming channel. The vibrator imposes fluctuations to the driving pressure and modulates the instantaneous flow rate of the perturbed phase, changing locally the diameters of the w/w jet. The shape of the corrugated jet can be controlled by applying appropriate frequency and amplitude of perturbation.
However, the perturbation approach has not been demonstrated in all-aqueous systems with dynamic viscosity of 100 mPa·s or above. An efficient breakup of a viscous w/w jet is restricted by the low growth rate of the Rayleigh-Plateau instability. Moreover, emulsions prepared using this approach are only stable for short periods of time due to coalescence of the dispersed phase.
There exists a need for methods of producing stable all-aqueous emulsions with high monodispersity, particularly emulsions with high viscosity (e.g., ≧100 mPa·s) and/or ultra-low interfacial tension.
Therefore, it is an object of the invention to provide methods of producing stable all-aqueous emulsions with high monodispersity, particularly emulsions with high viscosity (e.g., ≧100 mPa·s) and/or ultra-low interfacial tension.